“My guess is radiation.”
I’m guessing you are correct. Perhaps combined with other sensors. I was doing some work at a nuke testing facility and this guy would come out of the little lab building, get into his little Chevy and drive around a track with several sensors set up - poles on either side of the road.
He would go back into the little building and an hour later repeat the process.
I was finally near him at one point when I asked him what he was doing.
“Oh - I’m just running some tests on the various radiation sensors.” He then continued carrying his paper cup with the material in it and set it carefully in his cup holder (personal car!) Obviously dealing with VERY low levels.